1. Field of the Invention
The invention relates to a fuel injection amount control apparatus for an internal combustion engine, a control system for a power unit, and a fuel injection amount control method for an internal combustion engine. Particularly, the invention relates to a fuel injection amount control apparatus for an internal combustion engine, a control system for a power unit, and a fuel injection amount control method for an internal combustion engine which learn degradation of the injection performance of fuel injection valves of a vehicle-mounted internal combustion engine, and execute an actual fuel injection amount control according to the injection performance.
2. Description of the Related Art
Generally, internal combustion engines mounted in vehicles and, particularly, engines in which pressurized high-pressure fuel is injected by a high-response injector by a plurality of divided injection actions, such as recent diesel engines, are required to deliver good injection performance of causing the amount of fuel that is actually injected from the injector, that is, the actual injection amount, to accurately follow a target injection amount of a pilot injection or the like, which is a very small amount (i.e., the corresponding injection amount command value is a very small value). On the other hand, the injection accuracy of the injector with respect to the commanded injection amount, that is, the accuracy of the fuel injection amount control, gradually declines due to aging or time-dependent changes. Therefore, there has been developed an apparatus that grasps the degree of the time-dependent decline of the fuel injection accuracy of the injector by a learning process, and corrects the commanded injection amount to the injector so that a required actual injection amount is obtained.
As an example of a related-art fuel injection amount control apparatus of this kind for an internal combustion engine, there is known an apparatus constructed on the basis of a fact that in an idle rotation speed control (hereinafter, referred to as “ISC”), the injection amount command value is corrected so that an idling rotation speed can be maintained regardless of the time-dependent degradation of the injector, or the like, that is, an apparatus that learns the idle injection amount command value during ISC, and that curbs the decline in the accuracy of the fuel injection amount control that is caused by the time-dependent degradation of the injector, by correcting the injection amount command value during a normal operation by an amount corresponding to the time-dependent degradation of the injector on the basis of the learned value of the idle injection amount command value (e.g., see Japanese Patent Application Publication No. 2003-247447 (JP-A-2003-247447)). When an accessory load, such as the compressor of an air conditioner apparatus, or the like, is on, the foregoing control apparatus temporarily stops the driving of the accessory load. After executing the learning of the correction amount corresponding to the time-dependent degradation of the injector, the control apparatus restarts the accessory load.
It is known that the command value of the idle fuel injection amount remains a minimum value during a certain period of operation of the engine after being reduced as the engine friction decreases during the break-in operation period, and then the command value gradually increases as the injection efficiency of the injectors declines due to a long time of use. On the basis of this fact, a fuel injection amount control apparatus (e.g., see Japanese Patent Application Publication 2002-89333 (JP-A-2002-89333)) sets a difference between the foregoing minimum value as a reference value and the present average idle injection amount command value, as an index value that indicates the degree of time-dependent degradation of the injectors, and sets as a prerequisite condition for the learning the condition that the engine operation is in an idle stable state, the condition that the cooling water temperature is equal to or higher than a predetermined temperature, the condition that the air conditioner is off, the condition that the amount of fluctuation of the learned value of the idle injection amount command value is in a predetermined range without a change in the clutch engagement state or the like, the condition that the electric load is small, the condition that the elapsed time from the starting of the engine is longer than or equal to a certain length of time, the condition that the idle-up control is not being executed, the condition that the amount of fluctuation in engine rotation speed is within a predetermined range, etc. When a state in which the prerequisite condition is satisfied continues for a predetermined time or longer, the control apparatus calculates the difference that indicates the degree of decline in the injection accuracy.
There is known another fuel injection amount control apparatus (e.g., see Japanese Patent Application Publication No. 2005-36788 (JP-A-2005-36788)) that executes a learning-purpose injection with a very small amount of fuel during a specific state of the engine in which fuel injection is not executed, and then finds a difference in engine rotation speed between the case where the learning-purpose injection is executed and the case where the learning-purpose injection is not executed (the amount of rise in engine rotation speed caused by the learning-purpose injection), and accurately calculates the actual injection amount of fuel that was actually injected from the injector by the learning-purpose injection, on the basis of the amount of rise in engine rotation speed.
However, in the foregoing fuel injection amount control apparatus for an internal combustion engine or the control system for a power unit that includes the fuel injection amount control apparatus in the related art, since the relation between the engine rotation speed and the fuel injection amount tends to deteriorate due to variations among cylinders or fluctuations of accessory loads, it is not easy to perform a highly accurate injection amount control by correcting the injection amount command value for ordinary engine operation through the use of an injection amount correction value obtained during an idle rotation speed control. Moreover, in the foregoing related-art technologies, since the accessory loads are uniformly stopped from operating when the learning process for injection amount accuracy is executed, there is a problem of decline of drivability (meaning, in this application, not only the running performance of the vehicle but also the responsiveness of the vehicle side to commanding operations performed by a driver).
Besides, in the fuel injection amount control apparatus for an internal combustion engine in the related art which executes the learning-purpose injection of a very small amount of fuel during the specific vehicle operation state in which the engine has no fuel injection, the highly accurate learning of injection amount is possible, but that learning process can be executed only during the specific operation state in which the amount of rise in engine rotation speed caused by the learning-purpose injection of very small amount of fuel can be detected. Therefore, if a vehicle travel mode in which the engine operation state that allows the learning is unlikely to occur is set, it becomes difficult to promptly complete the learning process, so that the injection amount accuracy sometimes declines.
Concretely, for example, in the case where a lockup mechanism-equipped automatic transmission of a vehicle is completely locked-up, the rotation shaft of the transmission side is directly coupled to the engine, so that if the learning-purpose injection of very small amount of fuel is executed, the amount of rise in engine rotation speed cannot be accurately or appropriately determined. During an ordinary travel mode, the operation state of the vehicle is appropriately changed between an operation state in which the lockup mechanism is completely locked up and an operation state in which the lockup mechanism is not completely locked up (the torque converter slips) according to the state of travel of the vehicle. Therefore, the learning process can be executed when the completely locked-up state is not present. That is, as shown in FIG. 10A, since the degradation of an injector gradually progresses as the accumulated travel distance of the vehicle increases, periods for executing the learning process are set so that the learning process is executed before the injection amount accuracy of the injector reaches a certain permissible limit value Li (a line of accuracy shown by a dotted line in the diagram). In this manner, a required injection amount accuracy can be maintained. However, in a vehicle having an automatic transmission that is equipped with a manual shift function that enables a driver to perform shift operations to gear speeds or the like as the driver desires, the driver may sometimes continue to drive the vehicle in a manual shift mode in which the manual shift function is effective. Furthermore, in a vehicle having a lockup mechanism-equipped automatic transmission in which the complete lockup is executed in a quite low vehicle speed region for improved fuel economy or the like, the duration of the travel of the vehicle with the lockup mechanism being completely locked up can be considerably long. In such cases, the learning process cannot be completed within a certain period of time, and thus the opportunities of the learning decrease, so that the reliability of the learned value declines. Therefore, in the control apparatus in the related art, there is a possibility of failing to complete the learning process even after the fuel injection accuracy of the injector exceeds a permissible limit value Li as shown in FIG. 10B.
Thus, in the fuel injection amount control apparatuses for internal combustion engines and the control systems for a power unit that include the control apparatuses in the related art, the drivability is allowed to deteriorate in order to secure a certain time for the learning process, or while good drivability is secured, the learning time becomes insufficient, so that the injection amount accuracy declines. Thus, the related-art technologies cannot achieve both securement of good drivability and securement of good injection amount accuracy.